1. Field of the Invention
The present invention relates to an external-cavity wavelength tunable laser apparatus used for optical communications, and to a wavelength control method thereof.
2. Descriptions of Related Art
The transmission capacity of optical communications is acceleratingly increasing year after year. Wavelength division multiplex (WDM) communications have been realized as an economical, high-speed and large-capacity technique to meet the demand from the increase in the transmission capacity. WDM is a technique which simultaneously uses multiple beams (tens to one hundred wavelengths) of monochromatic light each with a wavelength not longer than a 0.01-μm interval, and which transmits mutually-different signals with their respective wavelengths. WDM is capable of increasing the transmission capacity for each fiber more than tens times. This makes it possible to cut back on costs for fiber installation to a large extent.
In a conventional practice, the light source for WDM needs: various types of semiconductor laser diodes with their respective wavelengths different from one another; and apparatuses, each constructed as a module, for driving the respective semiconductor laser diodes (hereinafter referred to as “modules”). When a semiconductor laser diode is intended to be manufactured, a crystal needs to be made for each wavelength, and a module is manufactured for each wavelength. This is problematic in terms of costs. Against this background, a wavelength tunable module has been developed, which includes a gain unit (light emitting unit) and a phase control region, and which is capable of freely tuning wavelengths. This type of module is capable of tuning optical wavelengths in a range of approximately −0.02 μm shorter than the center wavelength of the light emitting device to approximately +0.02 μm longer than the center wavelength. For this reason, it suffices that light emitting devices each with several wavelengths and corresponding modules are manufactured. It is possible to supply modules of this type economically. As a result, modules of this type present a chief method of realizing WDM.
Proceedings of the Institute of Electronics, Information and Communication Engineers International Conference Spring, 1991. C-149 has disclosed a technique for tuning wavelengths while maintaining a narrow spectrum by applying a positive or negative voltage to a DBR mirror having a diffraction grating in a DBR laser.
Japanese Patent Application Laid-open Publication No. 2006-245346 is an example of the related art.